Welding method, welding device, welded joint, and welded structure

ABSTRACT

A welding method is disclosed for butt welding a first base material  1  and a second base material  2  with each other. To also permit stable and efficient formation of a bead in the form of a fillet weld on a back side of a groove by conducting arc welding from the side of the groove without arranging a bead beforehand on the back side of the groove, the first member  1,  on which a root face  1   a  and a single groove face  1   b  have been formed, is brought at its root face  1   a  into contact with a flat surface  2   a  of the second base material  2.  A welding wire  6  is arranged facing on the groove formed by the contact between the first base material  1  and the second base material  2.  The first base material  1  and the second material  2  are caused to fuse at an area of contact between them by an arc from the welding wire  6.  A melt, which has been formed by the fusion at the area of contact and the like, is forced out to the back side of the groove to form a bead B 4  in the form of a fillet weld on the back side of the groove.

TECHNICAL FIELD

[0001] This invention relates to a welding method to be conducted byarranging a single groove face, a welding apparatus useful in thewelding method, and a welded joint and welded structure available by themethod.

BACKGROUND ART

[0002] In a variety of welded structures such as steel skeletonconstructions of buildings, bridges, industrial equipment andconstruction machinery, “T” joints are adopted as welded joints in manyinstances. In general, a “T” joint is formed by bringing a single grooveface of one of members into abutment against a surface of the othermember to arrange these members in a “T” pattern and forming a bead in agroove, which has been formed at an area of contact between the surfaceof the above-mentioned other member and the above-mentioned one member,to join these members together. As known examples of this type, thereare the welding methods disclosed in JP-A-08332567 and JP-A-06023544.The former conventional technique requires advance arrangement of a beadon the back side of a groove, which is formed at an area of contactbetween two members, to avoid burn-through of a groove root portion. Thelatter conventional technique, on the other hand, requires advancearrangement of a bead on the back of a groove, which is formed at anarea of contact between two members, to prevent the weld metal frompunching through or any part from remaining undeposited.

[0003] As a control method for making a center of a welding torch tracea weld line upon conducting automated welding, an arc sensor is widelyemployed to make use of electrical changes associated with changes inarc length and wire extension when the welding torch is caused to weavein the direction of the width of a groove. As conventional art of thissort primarily for an offset in the direction of the width of a groove,it is known, as disclosed in JP-B-53011502, to compare the magnitude ofan integral of welding currents obtained at left ends of weaving withthat of an integral of welding currents obtained at right ends of theweaving and then to correct the welding torch leftward or rightward onthe basis of a signal representative of the comparison to trace a weldline.

[0004] To prevent the burn-through of a groove root portion at an areaof contact between one of members, said one member having a singlegroove face, and the other member against which the single groovesurface is brought into abutment, the above-mentioned conventionaltechnique includes advance arrangement of a bead at the area of contacton a side opposite to the welded side. Depending on the shape of awelded joint structure or welded structure, however, it may bedifficult, from the standpoint of actually performing welding, toarrange a bead beforehand on the back side of a groove as in theabove-mentioned conventional technique.

[0005] When performing automated welding, an arc sensor is widelyemployed as in the above-mentioned conventional technique. Conventionalarc sensing methods, however, have difficulty in tracing a weld linebecause they are not designed to be applicable to such a welding methodthat by arc welding from the side of a groove, a bead is formed on theback side.

[0006] It is also difficult to trace a weld line when a move ordeformation occurs in a groove in the direction of the width of thegroove or in the direction of a distance to a tip of a welding wire dueto restraint conditions for a first base material and second basematerial or a thermal distortion or the like upon welding.

[0007] With the foregoing in view, work-related limitations areconsidered to be eliminated provided that a bead in the form of a filletweld can be stably formed on the back side of a groove by arc weldingfrom the side of the groove without arranging any bead beforehand on theback side of the groove.

[0008] The present invention has been completed in view of suchbackground circumstances, and its objects are to provide a weldingmethod for permitting stable and efficient formation of a bead in theform of a fillet weld on a back side of a groove by arc welding from theside of the groove without arranging a bead beforehand on the back sideof the groove, a welding apparatus useful for the welding method, and awelded joint and welded structure fabricated by the welding method.

DISCLOSURE OF THE INVENTION

[0009] To achieve the above-described objects, the present invention isconstituted as will be described in the following aspects.

[0010] Described specifically, a first aspect is characterized in thatin a welding method for butt welding a first base material and a secondbase material with each other, the welding method comprises bringing thefirst base material, on an end portion of which a root face and a singlegroove face have been formed, into contact at the root face thereof witha flat surface of the second base material, arranging a welding wire toface on a groove formed by the contact between the first base materialand the second base material, causing the first base material and thesecond material to fuse at an area of contact therebetween by an arcfrom the welding wire, and forcing a melt, which has been formed by thefusion, out to a back side of the groove to form a bead on the back sideof the groove.

[0011] According to the first aspect, a difference in thermal capacitybetween the first base material provided with the single groove face andthe second base material having the flat surface and fusion of thewelding wire make it possible to achieve welding in the groove and alsoto stably deposit a bead in the form of a fillet weld on the back sideof the first base material, said back side being on a side opposite tothe side of the groove. Described specifically, the bead in the form ofthe fillet weld can also be formed by causing a greater portion of thearc to generate on the side of the second material the thermal capacityof which is greater, causing the first base material, the thermalcapacity of which is smaller, to fuse to such an extent that the firstbase material is not punched through, and forcing out the melt of thewelding wire to the back side of the first base material, said back sidebeing on the side opposite to the side of the groove, while filling upthe groove with the melt of the welding wire. As no work is needed forthe arrangement of an auxiliary bead, a backing strip and the like, theworkability during welding can be improved, and moreover, themanufacturing cost can be lowered.

[0012] A second aspect is characterized in that in a welding method forbutt welding a first base material and a second base material with eachother, the welding method comprises positioning a center axis of an arcfrom a welding wire at an aimed point, which is located either at aposition of contact between a single groove face formed on the firstbase material and a flat surface of the second base material or at aposition adjacent and not farther than the position of contact,obliquely irradiating the arc at a predetermined inclination onto theflat surface of the second base material in a direction passing throughthe aimed point such that the first base material and the second basematerial are caused to fuse at an area of contact therebetween, andforcing a melt, which has been formed by the fusion, out to a back sideof the groove to form a bead on the back side of the groove.

[0013] According to the second aspect, the bead can be stably formedbecause the stability in the feeding of a voltage and current to thearea of contact can be enhanced.

[0014] A third aspect is characterized in that in the second aspect, thewelding wire is caused to weave with a predetermined oscillation widthwithin the groove while maintaining the predetermined inclination.

[0015] A fourth aspect is characterized in that in the second aspect,the welding wire is caused to weave with a predetermined oscillationwidth with a center of weaving set at a predetermined point on thepredetermined inclination within the groove.

[0016] According to the third or fourth aspect, stable welding isfeasible by the weaving operation of the welding wire.

[0017] A fifth aspect is characterized in that in the third or fourthaspect, while the welding wire is weaving, a change in welding voltageor welding current during a time after the arc moves past thepredetermined position until the arc moves toward the first basematerial and returns back to the position of contact is detected basedon a time at which the arc passes through the position of contact, andthe welding wire is moved in a direction of a width of the groove on abasis of the thus-detected change such that the center of the weavingalways remains at an appropriate position relative to the groove.

[0018] According to the fifth aspect, even if a move or deformationtakes place in the groove in the direction of the width of the groovedue to restraint conditions for the first base material and the secondbase material and/or a thermal distortion during welding, the weldingcan be appropriately corrected to trace a weld line in the groove bymoving the welding wire. This makes it possible to more accurately forceout the melt to the back side of the groove and hence, to also form abead in the form of a fillet weld.

[0019] A sixth aspect is characterized in that in the third or fourthaspect, while the welding wire is weaving, a change in welding voltageor welding current during a time after the arc moves past thepredetermined position until the arc moves toward the first basematerial and returns back to the position of contact is detected basedon a time at which the arc passes through the position of contact, and atip of the welding wire is moved toward or away from the position ofcontact on a basis of the thus-detected change such that the center ofweaving always remains at an appropriate position relative to thegroove.

[0020] According to the sixth aspect, even if a relative movement takesplace between the groove and a tip of the welding wire due to restraintconditions for the first base material and the second base materialand/or a thermal distortion or the like during welding and theirpositional relation is changed from their preset positional relation,the welding can be appropriately corrected to trace a weld line in thegroove by moving the tip of the welding wire toward or away from thegroove. This makes it possible to more accurately force out the melt tothe back side of the groove and hence, to also form a bead in the formof a good fillet weld.

[0021] A seventh aspect is characterized in that in the first or secondaspect, the bead formed on the back side of the groove is a weld bead ina form of a fillet weld.

[0022] According to the seventh aspect, a bead in the form of a filletweld can be stably formed.

[0023] An eighth aspect is characterized in that in the first or secondaspect, a current for generating the arc is a direct current.

[0024] A ninth aspect is characterized in that in the first or secondaspect, a current for generating the arc is a current obtained bysuperimposing a direct current and a pulse current on each other.

[0025] A tenth aspect is characterized in that in the first or secondaspect, a current for generating the arc is a sinusoidal current.

[0026] According to the ninth or tenth aspect, the directivity of thearc is improved and, even if a slight offset occurs between the weldingwire and the aimed point for a certain reason, the arc is not disturbedtoo much and the amount of a melt to be forced out to the side oppositeto the groove is stabilized. It is, therefore, possible to form a beadin the form of a fillet weld and to perform stable welding.

[0027] An eleventh aspect is characterized in that in a welded “T” jointwith a first base material and a second base material butt welded witheach other, the first base material and the second base material havebeen joined together at an area of contact between a flat surface of thesecond base material and the first base material, on which a root facepositioned in contact with the flat surface and a single groove face areformed, by causing the first base material and the second base materialto fuse at the area of contact with an arc from a welding wirepositioned facing on a groove formed by the single groove face and theflat surface and forcing a melt, which was formed by the fusion, out toa back side of the groove to form a bead in a form of a fillet weld.

[0028] A twelfth aspect is characterized in that in a butt-welded “T”joint of flat plates with a first base material and a second basematerial butt welded with each other, the first base material and thesecond base material have been joined together at an area of contactbetween an end surface of the second base material and the first basematerial, on which a root face positioned in contact with the endsurface and a single groove face are formed, by causing the first basematerial and the second base material to fuse at the area of contactwith an arc from a welding wire positioned facing on the groove andforcing a melt, which was formed by the fusion, out to a back sideopposite to a side of the groove to form a bead in a form of a filletweld.

[0029] A thirteenth aspect is characterized in that in a box-shaped,welded structure formed by butt welding a plurality of base materialsmade of plates, each two base materials butt welded with each other havebeen joined together at an area of contact between a flat surface of oneof the two base materials and the other base material, on which a rootface positioned in contact with the flat surface and a single grooveface are formed, by causing the base materials to fuse at the area ofcontact with an arc from a welding wire positioned facing on a grooveformed by the single groove face and the flat surface and forcing amelt, which was formed by the fusion, out to a back side of the one basematerial, said back side being on a side opposite to a side of thegroove, to form a bead in a form of a fillet weld.

[0030] According to the eleventh to thirteenth aspects, it is possibleto provide a welded joint or box-shaped structure which is low inmanufacturing cost and permits a reduction in manufacturing time.

[0031] A fourteenth aspect is characterized in that in a weldingapparatus for butt welding a first base material and a second basematerial with each other, the welding apparatus comprises a welding wireto be introduced into a groove formed at an area of contact between thefirst base material and the second material, a control device forcontrolling a position of the welding wire, a welding power source forfeeding welding electric power to the area of contact, and control meansfor outputting a command to the control device such that the weldingwire is moved toward the second base material when an actual voltage fedfrom the welding power source to the area of contact has exceeded afirst preset voltage level set in advance and the welding wire is movedtoward the first base material when an actual current fed from thewelding power source to the area of contact has exceeded a presetcurrent level set in advance or the actual voltage has fallen below asecond preset voltage level set in advance.

[0032] According to the fourteenth aspect, while performing arc weldingof the first base material and the second base material from one side,the center axis of an arc can be accurately position-controlled at apredetermined inclination relative to the second base material so that abead in the form of a fillet weld can be stably formed on the back sideof the groove side.

[0033] A fifteenth aspect is characterized in that in a weldingapparatus for butt welding a first base material and a second basematerial with each other, the welding apparatus comprises a welding wireto be introduced into a groove formed at an area of contact between thefirst base material and the second material, a control device forcontrolling a position of the welding wire, a welding power source forfeeding welding electric power to the area of contact, and control meansfor outputting a command to the control device such that the weldingwire is moved toward the second base material when an actual voltage fedfrom the welding power source to the area of contact has exceeded afirst preset voltage level set in advance and the welding wire is movedtoward the first base material when an actual current fed from thewelding power source to the area of contact has exceeded a presetcurrent level set in advance or the actual voltage has fallen below asecond preset voltage level set in advance and also for comparing theactual current and voltage with a preset average current level andpreset average voltage level set in advance, respectively, andoutputting a current control signal and voltage control signal to thewelding power source such that these actual current and voltage becomeequal to the preset current and voltage levels set in advance.

[0034] A sixteenth aspect is characterized in that in a weldingapparatus for butt welding a first base material and a second basematerial with each other, the welding apparatus comprises a weldingwire, a welding torch for holding the welding wire in place, feedingmeans for feeding the welding wire to the welding torch, a control unitprovided with a position control device for positionally controlling thewelding wire in directions of three orthogonal axes, a welding powersource for feeding welding electric power to an area of contact betweenthe first base material and the second base material, an average voltagesetting device for setting an average voltage level for the welding, anaverage current setting device for setting an average current level forthe welding, a voltage detector and current detector for detecting anactual welding voltage and actual welding current to form, by thewelding wire positioned facing on a groove formed by a single grooveface arranged on the first base material and a flat surface of thesecond base material, a bead in a form of a fillet weld on a back sideof the groove, a power diagnosis unit for being inputted with the actualvoltage detected by the voltage detector and the actual current detectedby the current detector, outputting a command to the position controldevice such that the welding wire is moved toward the second basematerial when the actual voltage has exceeded a first preset voltagelevel set in advance and the welding wire is moved toward the first basematerial when the actual current detected by the current detector hasexceeded a preset current level set in advance or the actual voltagedetected by the voltage detector has fallen below a second presetvoltage level set in advance, and also outputting the actual voltage andoutputting the actual current, the current control device for comparingthe average current level inputted from the average current settingdevice with the actual current inputted from the power diagnosis unitand outputting a current control signal such that this actual currentbecomes equal to the preset current level set in advance, a voltagecontrol device for comparing the average voltage level inputted from theaverage voltage setting device with the actual voltage inputted from thepower diagnosis unit and outputting a voltage control signal such thatthis actual voltage becomes equal to the preset voltage level set inadvance, and the output control device for being inputted with thecurrent control signal outputted from the current control device and thevoltage control signal outputted from the voltage control device, andbased on these current control signal and voltage control signal,outputting a power source control signal to the welding power source tocontrol an output of the power source.

[0035] According to the fifteenth or sixteenth aspect, while performingarc welding of the first base material and the second base material fromone side, the center axis of an arc is accurately position-controlled ata predetermined inclination relative to the second base material, and anaverage voltage and average current are also controlled to become equalto their corresponding preset values. It is, therefore, possible tostably form a bead in the form of a fillet weld on the back side of thegroove.

[0036] A seventeenth aspect is characterized in that in the sixteenthaspect, the welding apparatus is provided with a peak voltage settingdevice for setting a peak voltage for a pulse voltage, a base voltagesetting device for setting abase voltage, a pulse duration settingdevice for setting a pulse duration, and a pulse waveform control devicefor being inputted with the preset peak voltage, preset base voltagelevel and preset pulse duration from the peak voltage setting device,base voltage setting device and pulse duration setting device,respectively, comparing these preset values with an actual peak voltage,actual base voltage and actual pulse duration, respectively, andoutputting a waveform control signal to the output control device, thepower diagnosis unit comprises means for determining an actual peakvoltage, actual base voltage and actual pulse duration on a basis of theactual voltage detected by the voltage detector and the actual currentdetected by the current detector, and outputting the thus-determinedactual peak voltage, actual base voltage and actual pulse duration tothe pulse waveform control device, and the output control devicecomprises means for being inputted with the waveform control signaloutputted from the pulse waveform control device and outputting a powersource control signal, which controls an output of the power source, tothe welding power source on a basis of the voltage control signalinputted from the voltage control device and the current control signalinputted from the current control device.

[0037] An eighteenth aspect is characterized in that in the sixteenthaspect, the pulse waveform control device comprises sinusoidal currentforming means for converting a welding current into a sinusoidalcurrent, comparing a preset sinusoidal waveform, which has been set inadvance, with an actual sinusoidal waveform and outputting a waveformcontrol signal to the output control device, the power diagnosis unitcomprises means for determining an actual current waveform on a basis ofthe actual voltage detected by the voltage detector and the actualcurrent detected by the current detector, and outputting thethus-determined actual current waveform to the sinusoidal currentforming means, and the output control device comprises means foroutputting a power source control signal, which controls an output ofthe power source, to the welding power source on a basis of the waveformcontrol signal inputted from the sinusoidal current forming means, thevoltage control signal inputted from the voltage control device and thecurrent control signal inputted from the current control device.

[0038] According to the seventeenth or eighteenth aspect, a pulsecurrent is used as an arc welding current so that an arc is providedwith higher directivity and can more precisely impinge on a positionaimed by the arc. Further, the arc can be accurately maintained at apredetermined inclination. This makes it possible to prevent occurrenceof an inconvenience such as a burn-through at the area of contactbetween the first base material and the second base material orremaining of undeposited parts, and accordingly, to deposit afillet-weld-shaped bead in a stable form on the back side of the groove.

[0039] A nineteenth aspect is characterized in that in a weldingapparatus for butt welding a first base material and a second basematerial with each other, the welding apparatus comprises a welding wireto be introduced through a welding torch into a groove formed bybringing the first base material, on which a root face and a singlegroove face have been formed, into contact with the second materialhaving a flat surface, a control device for controlling a position ofthe welding wire, a welding power source for feeding welding electricpower to the area of contact, a voltage detector for detecting an actualvoltage to be fed from the welding power source to the area of contact,a clock device for detecting a time, at which an arc from the weldingwire passes through a position of contact between an end portion of thesingle groove face and the flat surface, and clocking a time after thearc passes through the position of contact until the arc moves towardthe first base material and returns back to the predetermined position,memory means for storing a change in welding voltage during the clockingtime by the voltage detector and the clock device, and wire positioncontrol means for continuing preceding weaving control when a time(t₄−t₃)−t₃ being a time at which an actual voltage begins to exceed apreset voltage level set in advance and t₄ being a time at which theactual voltage falls below the preset voltage level, both while clockinga second predetermined time Ät₂ subsequent to an elapse of a firstpredetermined time Ät₁ from a time point at which the welding wirepassed through the position of contact during the clocking time—fallswithin a target time range in which adequate fusion takes place withoutpunching through the first base material, moving a center axis ofweaving of the welding wire toward the second base material when thetime (t₄−t₃) exceeds the target time, and moving the center axis ofweaving toward the first base material when the time (t₄−t₃) falls belowthe target time.

[0040] A twentieth aspect is characterized in that in a weldingapparatus for butt welding a first base material and a second basematerial with each other, the welding apparatus comprises a welding wireto be introduced through a welding torch into a groove formed bybringing the first base material, on which a root face and a singlegroove face have been formed, into contact with the second materialhaving a flat surface, a control device for controlling a position ofthe welding wire, a welding power source for feeding welding electricpower to the area of contact, a current detector for detecting an actualcurrent to be fed from the welding power source to the area of contact,a clock device for detecting a time, at which an arc from the weldingwire passes through a position of contact between an end portion of thesingle groove face and the flat surface, and clocking a time after thearc passes through the position of contact until the arc moves towardthe first base material and returns back to the predetermined position,memory means for storing a change in welding current during the clockingtime by the current detector and the clock device, and wire positioncontrol means for continuing preceding weaving control when a time(t₄−t₃)−t₃ being a time at which an actual current begins to fall belowa preset current level set in advance and t₄ being a time at which theactual current exceeds the preset voltage level, both while clocking asecond predetermined time Ät₂ subsequent to an elapse of a firstpredetermined time Ät₁ from a time point at which the welding wirepassed through the position of contact during the clocking time—fallswithin a target time range in which adequate fusion takes place withoutpunching through the first base material, moving a center axis ofweaving of the welding wire toward the second base material when thetime (t₄−t₃) exceeds the target time, and moving the center axis ofweaving toward the first base material when the time (t₄−t₃) falls belowthe target time.

[0041] According to the nineteenth or twentieth aspect, even if amovement or distortion takes place in the groove in the direction of thewidth of the groove due to restraint conditions for the first basematerial and the second base material and/or a thermal distortion or thelike during welding, the welding can be appropriately corrected to tracea weld line in the groove by moving the welding wire on the basis of theactual voltage or actual current detected by the actual voltage detectoror actual current detector. This makes it possible to more accuratelyforce out the melt to the back side of the groove and hence, to alsoform a bead in the form of a good fillet weld. As no work is needed forthe arrangement of an auxiliary bead, a backing strip or the like andthe welding can be performed by automatically and more accuratelytracing the weld line, the workability of the welding can be improved.

[0042] A twenty-first aspect is characterized in that in a weldingapparatus for butt welding a first base material and a second basematerial with each other, the welding apparatus comprises a welding wireto be introduced through a welding torch into a groove formed bybringing the first base material, on which a root face and a singlegroove face have been formed, into contact with the second materialhaving a flat surface, a control device for controlling a position ofthe welding wire, a welding power source for feeding welding electricpower to the area of contact, a voltage detector for detecting an actualvoltage to be fed from the welding power source to the area of contact,a clock device for detecting a time, at which an arc from the weldingwire passes through a position of contact between an end portion of thesingle groove face and the flat surface, and clocking a time after thearc passes through the position of contact until the arc moves towardthe first base material and returns back to the predetermined position,memory means for storing a change in welding voltage during the clockingtime by the voltage detector and the clock device, and wire tip positioncontrol means for moving a tip of the welding wire away from theposition of contact when an actual average voltage during the clockingtime falls below a preset first voltage level, moving the tip of thewelding wire toward the position of contact when the actual averagevoltage exceeds a preset second voltage level, and maintaining the tipof the welding wire at a preceding position when the actual averagevoltage falls between the preset first voltage level and the presetsecond voltage level.

[0043] A twenty-second aspect is characterized in that in a weldingapparatus for butt welding a first base material and a second basematerial with each other, the welding apparatus comprises a welding wireto be introduced through a welding torch into a groove formed bybringing the first base material, on which a root face and a singlegroove face have been formed, into contact with the second materialhaving a flat surface, a control device for controlling a position ofthe welding wire, a welding power source for feeding welding electricpower to the area of contact, a current detector for detecting an actualcurrent to be fed from the welding power source to the area of contact,a clock device for detecting a time, at which an arc from the weldingwire passes through a position of contact between an end portion of thesingle groove face and the flat surface, and clocking a time after thearc passes through the position of contact until the arc moves towardthe first base material and returns back to the predetermined position,memory means for storing a change in welding current during the clockingtime by the current detector and the clock device, and wire tip positioncontrol means for moving a tip of the welding wire away from theposition of contact when an actual average current during the clockingtime exceeds a preset first current level, moving the tip of the weldingwire toward the position of contact when the actual average currentfalls below a preset second current level, and maintaining the tip ofthe welding wire at a preceding position when the actual average currentfalls between the preset first current level and the preset secondcurrent level.

[0044] According to the twenty-first or twenty-second aspect, even if amovement or distortion takes place in the groove in the direction of thewidth of the groove due to restraint conditions for the first basematerial and the second base material and/or a thermal distortion or thelike during welding, the welding can be appropriately corrected to tracea weld line in the groove by moving the position of the tip of thewelding wire closer or away on the basis of the actual average voltageor actual average current detected by the actual voltage detector oractual current detector. This makes it possible to more accurately forceout the melt to the back side of the groove and hence, to also form abead in the form of a good fillet weld. As no work is needed for thearrangement of an auxiliary bead, a backing strip or the like and thewelding can be performed by automatically and more accurately tracingthe weld line, the workability of the welding can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045]FIG. 1 is a schematic view illustrating welding work for a “T”joint by a first embodiment of the present invention.

[0046]FIG. 2 is a perspective view showing the “T” joint welded by thewelding work illustrated in FIG. 1.

[0047]FIG. 3 is a schematic view showing a state in which the positionof a welding wire has been set before initiation of welding in thewelding work illustrated in FIG. 1.

[0048]FIG. 4 is a schematic view depicting a state in which the weldinghas been initiated in the state shown in FIG. 3.

[0049]FIG. 5 is a schematic view showing a state in which the weldinghas proceeded further from the state depicted in FIG. 4.

[0050]FIG. 6 is a schematic view illustrating an example of butt weldingwork on plates having different thicknesses in accordance with Example 3of a first embodiment of the present invention.

[0051]FIG. 7 is a schematic view illustrating another example of thebutt welding work on plates having different thicknesses in accordancewith Example 3 of the first embodiment of the present invention.

[0052]FIG. 8 is schematic view showing a modification of a weldablegroove shape in the first embodiment of the present invention.

[0053]FIG. 9 is schematic view showing another modification of theweldable groove shape in the first embodiment of the present invention.

[0054]FIG. 10 is a schematic view illustrating welding work for a “T”joint by a second embodiment of the present invention.

[0055]FIG. 11 is a schematic view illustrating welding work for a “T”joint by a third embodiment of the present invention.

[0056]FIG. 12 is a front elevation of a hydraulic excavator equippedwith welded structures fabricated by a welding method according to afourth embodiment of the present invention.

[0057]FIG. 13 is a cross-sectional view taken along XIII-XIII of FIG.12.

[0058]FIG. 14 is a cross-sectional view taken along XIV-XIV of FIG. 12.

[0059]FIG. 15 is a cross-sectional view depicting a butt-welded jointbetween upper plates of different thicknesses in a boom of the hydraulicexcavator, said butt-welded joint being designated at sign XV in FIG.12.

[0060]FIG. 16 is a diagram showing the construction of a weldingapparatus according to a fifth embodiment of the present invention.

[0061]FIG. 17 is a schematic view illustrating an example of weavingoperation of a welding wire in the welding apparatus shown in FIG. 16.

[0062]FIG. 18 is a diagram showing a relationship between the weavingposition of the welding wire and the welding voltage generated in thewelding wire under such welding conditions that minimize the forcing-outof a melt to the back side of a groove by the welding apparatus shown inFIG. 16.

[0063]FIG. 19 is a diagram showing a relationship between the weavingposition of the welding wire and the welding voltage generated in thewelding wire under such welding conditions that maximize the forcing-outof a melt to the back side of a groove by the welding apparatus shown inFIG. 16.

[0064]FIG. 20 is a flow chart illustrating a control procedure for theweaving position of the welding wire in the welding apparatus shown inFIG. 16.

[0065]FIG. 21 is a flow chart illustrating a continuation of the controlprocedure of FIG. 20.

[0066]FIG. 22 is a diagram showing the construction of a weldingapparatus according to a sixth embodiment of the present invention.

[0067]FIG. 23 is a waveform diagram of a pulse current in the weldingapparatus shown in FIG. 22.

[0068]FIG. 24 is a diagram illustrating, as a graph, a relationshipbetween the leg length of a bead in the form of a fillet weld as formedon the back side of a groove and the aimed point of a welding apparatusto which a welding method according to the present invention is applied.

BEST MODES FOR CARRYING OUT THE INVENTION

[0069] A description will hereinafter be made about embodiments of thepresent invention.

[0070] 1. First Embodiment

[0071]FIG. 1 through FIG. 9 are drawings for illustrating a weldingmethod according to a first embodiment of the present invention.

[0072]FIG. 1 is a schematic view illustrating welding work for a “T”joint by the first embodiment of the welding method of the presentinvention, and FIG. 2 is a perspective view showing the “T” joint weldedby the welding work illustrated in FIG. 1.

[0073]FIG. 1 illustrates a state upon welding a first base material 1and a second base material 2 with each other by bringing them intoabutment in the form of “T”. The first base material 1 is provided at abutted end portion thereof with a root face 1 a and a single groove face1 b which serves to form a single bevel groove. The second base material2 has a flat surface 2 a positioned in contact with the root face 1 a ofthe first base material 1, and in combination with the first basematerial 1, forms a “T” joint. Numeral 5 designates a welding torch, andnumeral 6 indicates a welding wire held in place by the welding torch 5and fed by unillustrated feeding means. In FIG. 1, there are alsoindicated a dimension Rf in the direction of the thickness of the rootface 1 a, a position P₁ of contact between an end portion of the singlegroove face 1 b of the first base material 1 and the flat surface 2 a ofthe second base material 2, a point P₂ aimed by the center axis of anarc, and a distance d from the contact position P₁ to the aimed point P₂by the center axis of the arc. Also shown are a groove angle è, apredetermined inclination ä between the center axis of the arc (weldingwire) and the flat surface 2 a of the second base material 2, and thecenter axis A of the arc.

[0074] With reference to the drawings, a description will hereinafter bemade based on specific examples.

1.2 FIRST EXAMPLE

[0075] In this example, the first and second base materials 1,2 weremade of steel, and the dimension Rf of the root face 1 a was 1 mm. Bysetting the root gap at 0 mm (in other words, the root face was in acontacted position) and the groove angle è at 45 degrees, CO₂ arcwelding was conducted with the welding apparatus. As welding conditions,the average welding current was set at 310 A, the average weldingvoltage at 33 V, and the moving speed at 35 to 40 cm/min.

[0076] Under the above conditions, it was possible to stably deposit abead in the form of a fillet weld of 2 to 8 mm at an area of abutmentbetween a side 1 c of the first base material 1, said side being locatedon a side opposite to the groove, and the flat surface 2 a of the secondbase material 2 when the aimed point P₂ by the welding wire 6 (thecenter axis of the arc) was set 0 to 3 mm apart from the contactposition P₁, the second base material 2 was placed in a substantiallyhorizontal position, and the predetermined inclination a formed betweenthe flat surface 2 a and the welding wire 6 was set at 15 to 35 degrees.

[0077] The results of the welding are shown in FIG. 2. FIG. 2illustrates a bead B4 deposited in the form of a fillet weld between theside 1 c of the base material 1, said side being located on a sideopposite to the groove, and the flat surface 2 a of the second basematerial 2, a bead B5 resolidified while causing the flat surface 2 a ofthe second base material 2 to fuse, and a bead B6 formed on the side ofthe groove with addition of a melt from fusion of the welding wire 6itself subsequent to the formation of the bead B4.

[0078] It is to be noted that the above-described specific values shouldbe determined by taking into consideration various conditions such asthe plate thicknesses of the first and second base materials 1,2 and theshape of the area of abutment between the first base material 1 and thesecond base material 2.

[0079] The process of formation of the beads on the “T” joint in thisexample is shown in FIG. 3 to FIG. 5.

[0080]FIG. 3 is a schematic view showing a state in which the positionof the welding wire has been set before initiation of welding in thewelding work illustrated in FIG. 1. As illustrated in FIG. 3, thewelding wire 6 is positioned opposite the groove by setting the aimedpoint P₂ of the welding wire 6 at a point a little apart toward thewelding direction from the position P₁ of contact between the endportion of the groove face 1 b and the flat surface 2 a and tilting thedirection of the welding wire at the predetermined inclination arelative to the flat surface 2 a of the second base material 2 thethermal capacity of which is greater.

[0081]FIG. 4 is a schematic view depicting a state in which welding hasbeen initiated in the above-mentioned state shown in FIG. 3. Because thewelding wire 6 is directed toward the flat surface 2 a as depicted inFIG. 4, more heat is inputted in the flat surface 2 a than in the singlegroove face 1 b the thermal capacity of which is smaller. While bothbase materials 1,2 are being caused to fuse in a well-balanced manner, amolten pool is hence formed with addition of a melt from fusion of thewelding wire 6 itself. As a result, on the side of the first basematerial 1 having the single groove face 1 b of smaller thermalcapacity, a fusion zone reaches the back side 1 c which is located onthe back side of the groove.

[0082]FIG. 5 is a schematic view showing a state in which the weldinghas proceeded further from the state depicted in FIG. 4. As shown inFIG. 5, the molten pool is forced out to the side opposite to thewelding side, i.e., to the back side of the first base material 1 by theforce of the arc, convection in the molten pool, and the like, and underthe action of surface tension and the like, a molten pool is formedextending integrally from the inside of the groove to the back side ofthe base material 1. The melt then resolifies so that beads B4, B5 andB6 such as those shown in FIG. 2, namely, a first layer of the “T” jointis formed.

[0083] The above-described welded state corresponds to the welding of afirst layer in the groove. Depending on the size of the groove, thewelding of the remaining portion in the groove can be conducted in asimilar manner as in the conventional art, and therefore, itsdescription is omitted.

1.2 SECOND EXAMPLE

[0084] As a second example, the following example can be given.

[0085] Under conditions that CO₂ arc welding was chosen as a weldingprocess, the root face Rf was set at 1 mm, the root gap was set at 0 mm,the groove angle è was set at 45 degrees, the average current was set at310 A, the average voltage was set at 33 V, the welding speed was set at35 cm/min, the aimed point P₂ of the welding wire 6 was set at a point 2mm apart from the contact position P₁ (d=2 mm), and the inclination awas set at 25 degrees, a “T” joint was welded from the side of thegroove. When welded as described above, a bead B4 in the form of afillet weld, which had a leg length of 7 mm in the vertical direction(on the side of the first base material 1) and a leg length of 5 mm inthe horizontal direction (on the side of the second base material 2),was successfully deposited between the back side 1 c of the first basematerial 1, said back side being located on the back side of the groove,and the flat surface 2 a of the second base material 2 without leavingany undeposited part in the groove. It was also possible to obtain abead B4 in the form of a fillet weld in a relatively stable manner bysetting the aimed point P₂ of the welding wire 6 at a position 2 to 3 mmapart from the contact position P₁.

1.3 THIRD EXAMPLE

[0086] A third example is shown in FIG. 6 and FIG. 7.

[0087]FIG. 6 and FIG. 7 are drawings, each of which illustrates thethird example in which two plates having different thicknesses arebutt-welded. In FIG. 6, a first base material 1′ consisting of a plateand a second base material 2′ having a greater thickness than the firstbase material 1′ are kept in abutment against each other with one sidesthereof being arranged on the same plane. The drawing also shows a rootface 1′a of the base material 1′, a groove 1′b arranged on one side ofan abutted portion of the base material 1′, a side 1′c of the basematerial 1′, said side being located on a side opposite to the groove1′b, and a flat surface 2′a of the second base material 2′. In thebutt-welded joint shown in FIG. 6, a bead B4′ can also be deposited atan area, where the joint is desired, to form the joint even by one-sidewelding from the side of the groove if the aimed point of the weldingtorch 5, i.e., the welding wire 6, the relationship of the welding torch5, i.e., the welding wire 6 with the groove, and the like are set asconditions as illustrated in FIG. 1 which has been described above.

[0088] In the example of FIG. 7 in which the two plates are butt-welded,both of the base materials 1′,2′ are arranged with one sides thereofextending in parallel with each other, and the thinner one of both ofthe base materials 1′,2′, that is, the first base material 1′ ispositioned adjacent a center of the thicker, second base material 2′ asviewed in the direction of its thickness such that they are kept inabutment against each other.

[0089]FIG. 7 is similar to the above-mentioned example illustrated inFIG. 6, but is different only in the final bead shape on the side of thegroove in the welded joint. Accordingly, a description of the example ofFIG. 7 is omitted. In the example of FIG. 7, the welding only from theside of the groove, in other words, from the one side can also form thewelded joint as a joint which permits well-balanced transmission offorce owing to the registration between the centers of the plates asviewed in the directions of their thicknesses.

1.4 FOURTH EXAMPLE

[0090] Other examples of the groove shape weldable by the welding methodof the present invention will next be shown in FIG. 8 and FIG. 9.

[0091]FIG. 8 shows an example in which a J-shaped, single groove face 1b 1 is arranged on a first base material 1, while FIG. 9 illustrates anexample in which a single groove face 1 b 2 is arranged with pluralstraight lines. In each of these examples, a desired root face 1 a isarranged. Similarly to the above-mentioned case of the single bevelgroove, a bead B4 in the form of a fillet weld can, therefore, be formedby arc welding from one side between a back side 1 c of the first basematerial 1, said back side being located on the back side of the groove,and a flat surface 2 a of a second base material 2 even if the grooveshape is other than a single bevel groove.

[0092] 1.5 Aimed Point P₂ and the Results of Welding

[0093]FIG. 24 is a diagram illustrating, as a graph, a relationshipbetween the leg length of a bead in the form of a fillet weld as formedon the back side of a groove and the aimed point of a welding apparatusto which a welding method according to the present invention is applied.

[0094] As shown in FIG. 24, the leg length of the bead B4 to be formedbetween the back side 1 c of the first base material 1, said back sidebeing located on the back side of the groove, and the flat surface 2 aof the second base material 2 is formed in a range of from about 4 to 7mm in the vertical direction when the distance d between the contactposition P₁ and the aimed point P₂ is in a range of from 2 to 5 mm. Evenwhen the distance d between P₁ and P₂ exceeds 4 mm, the bead B4 canstill be formed between the back side 1 c of the first base material 1and the flat surface 2 a of the second base material 2 although the leglength of the bead B4 is undesirably shortened to 1 mm or so in thehorizontal direction. Further, this bead B4 is formed with a desiredhorizontal leg length of about 4 mm or so when the distance d between P₁and P₂ is in a range of from 2 to 3 mm.

[0095] As described above, owing to the difference in thermal capacitybetween the first base material with the single groove face 1 b arrangedthereon and the second base material having the flat surface 2 a and thefusion of the welding wire, this embodiment makes it possible to weldthe groove and also to stably deposit the bead B4 in the form of afillet weld between the back side 1 c of the first base material 1, saidback side being located on the back side of the groove, and the flatsurface 2 a of the second base material 2.

[0096] Described specifically, a greater portion of an arc is caused togenerate on (a greater portion of the heat of an arc is caused to betransferred to) the side of the second base material 2 of larger thermalcapacity. On the side of the single groove face 1 b of the first basematerial 1 of smaller thermal capacity, the base material 1 is caused tofuse wholly to the back side 1 c on the back side of the groove to suchan extent that no punch-through takes place, and further, fusion of thewelding wire 6 forces a melt out to the back side of the groove whilefilling up the groove. The bead B4 in the form of the fillet weld can beformed accordingly.

[0097] If there is a further part to be welded in another groove, itswelding in a similar manner as in conventional art makes it possible toform a desired “T” joint.

[0098] On the inner side of a box-shaped, welded structure, beads in theform of fillet welds can, therefore, be deposited in a groove and on theback side of the groove, respectively, by simply arranging the groove onthe outer side of a joint to be butt-welded and conducting arc weldingfrom the side of the groove. As a consequence, a joint can be fabricatedwhile retaining the strength, and no work is needed to arrange a bead inadvance and hence, the efficiency of the welding work can besignificantly improved.

[0099] Even in such a structure that includes such a joint part aspermitting no welding from the back side of a groove and thusunavoidably requiring use of thicker base materials for being to bewelded together, a bead in the form of a fillet weld can also bedeposited on the back side of the groove by performing arc welding onlyfrom one side without leaving any part undeposited in the groove. It is,therefore, possible to reduce the thicknesses of base materials withouta reduction in the strength of the resulting structure.

[0100] 2. Second Embodiment

[0101]FIG. 10 is a schematic view illustrating a second embodiment ofthe welding method according to the present invention, and shows weldingwork for a “T” joint. In this embodiment, elements equivalent to thecorresponding elements in the above-described first embodiment aredesignated by like reference signs, and overlapping descriptions areomitted.

[0102] In this second embodiment, a welding wire 6 held in place on awelding torch 5 and fed by unillustrated feeding means is held at aninclination a relative to a flat surface 2 a of a second base material 2while being directed to an aimed point P₂. In this embodiment, weldingis performed while causing the welding wire 6 to weave. The weaving iseffected by driving the welding torch 5 under control such that thewelding wire 6 oscillates at a predetermined amplitude in the directionof an arrow 50 while maintaining the above-described inclination ä. Theamplitude and pitch of oscillation for the weaving are determineddepending on the thickness of a first base material 1, the size of agroove, etc.

[0103] Even when welding is performed while causing the welding wire 6to weave as described above, an arc is directed more toward the side ofthe flat surface 2 a of the second base material 2 of greater thermalcapacity while the predetermined inclination ä is always maintained. Asthe oscillation is effected in this state, a molten pool is forced outto the back side of the groove so that a bead B4 in the form of a filletweld can be deposited between the back side 1 c of the first basematerial 1, said back side being located on the back side of the groove,and the flat surface 2 a. In this manner, efficient welding is feasibleeven where the groove angle è is rather large.

[0104] As an alternative, the welding wire 6 can also be caused to weavein the direction of its axis as indicated by an arrow 60 in the samedrawing, that is, FIG. 10. In this case, a bead B4 in the form of afillet weld can also be caused to deposit between the back side 1 c ofthe first base material 1, said back side being located on the back sideof the groove, and the flat surface 2 a of the second base material 2 byforcing out a molten pool to the back side of the groove under arc forceor the like.

[0105] 3. Third Embodiment

[0106]FIG. 11 is a schematic view illustrating a third embodiment of thewelding method according to the present invention, and shows weldingwork for a “T” joint. In this embodiment, elements equivalent to thecorresponding elements in the first and second embodiments aredesignated by like reference signs, and overlapping descriptions areomitted.

[0107] In the third embodiment, weaving of a welding wire 6 is performedby driving a welding torch 5 under control such that a tip of a weldingwire 6 is swung in the form of a circular arc over á degrees about apredetermined inclination ä as a center. In this third embodiment, abead B4 in the form of a fillet weld can also be deposited between aback side 1 c of a first base material 1, said back side being locatedon the back side of the groove, and a flat surface 2 a as in theabove-mentioned embodiments.

[0108] 4. Fourth Embodiment

[0109]FIG. 12 is a front elevation of a hydraulic excavator as a weldedstructure fabricated by the welding method according to the presentinvention.

[0110] In FIG. 12, the hydraulic excavator comprises a travel base 70,an upperstructure 71 mounted for revolution on the travel base 70, and arevolving mechanism 72 for causing the upperstructure 71 to revolve.This revolving mechanism 72 can cause the upperstructure 71 to revolveover 360 degrees relative to the travel base 70. The travel base 70 isprovided with a travel track frame, drive wheels arranged on the traveltrack frame, crawlers wrapped on the drive wheels, etc., and allows thehydraulic excavator to travel.

[0111] The upperstructure 71 has an unillustrated revolving frame, andis provided with various hydraulic equipment such as an engine andhydraulic pumps, an operator's cab 73, etc. By operating control devicesarranged in the operator's cab 73, an operator can drive the travel base70, working equipment 80 to be described subsequently herein, and thelike. The working equipment 80 is generally constructed of a boom 74arranged pivotally up and down on the upperstructure 71, a boom cylinder75 for driving the boom 74, an arm 76 pivotally arranged on the boom 74,an arm cylinder 77 for driving the arm 76, a bucket 78 attached to thearm 76, a bucket cylinder 79 for driving the bucket 78, and the like.

[0112] Elements making up the above-described hydraulic excavator, suchas the track frame, the revolving frame, the boom 74 and the arm 76, arewelded structures formed by combining welded joints typified by “T”joints. As their cross-sectional shapes are approximately box-shaped inmany instances, it is difficult to arrange auxiliary beads, backingstrips or the like on their back sides. The present invention can,however, overcome this difficulty.

[0113]FIG. 13 is a cross-sectional view taken along XIII-XIII of FIG.12, FIG. 14 is a cross-sectional view taken along XIV-XIV of FIG. 12,and FIG. 15 is a cross-sectional view of a part designated at sign XV inFIG. 12. More specifically, FIG. 13 is across-sectional view of the boom74 of the hydraulic excavator in FIG. 12, FIG. 14 is a cross-sectionalview of the arm 76 of the hydraulic excavator in FIG. 12, and is across-sectional view showing a butt-welded joint part of upper platesthemselves of different thicknesses in the boom of the hydraulicexcavator. These boom 74 and arm 76 are in the forms of box-shaped,welded structures, and in FIG. 15, a butt-welded joint between a thickplate member 74 a′ and a thin plate member 74 a″, both of which make upan upper plate of the boom 74, is formed.

[0114] The boom 74 is constructed of an upper plate 74 a, a left andright side plates 74 b,74 b, and a lower plate 74 c. Upon forming theboom 74 by applying the welding method of the present invention, eachside plate 74 b with a root face formed thereon and also with a singlegroove face arranged on an outer side thereof is brought into abutmentagainst the lower plate 74 c having a flat surface, and welding work isperformed from the side of the groove by using the above-mentionedwelding method shown in FIG. 1. This welding work can form a bead 74 gin the groove between the single groove face of the side plate 74 b andthe flat surface of the lower plate 74 c, and can also deposit a beadB74 in the form of a fillet weld between the inner wall of the sideplate 74 b and the flat surface of the lower plate 74 c. According tothis embodiment, a box-shaped, welded structure welded also on the innerside of the structure can be fabricated by welding work only from theoutside of the structure without needing arranging a backing strip orarranging a bead for the prevention of a burn-through, a punch-throughor the like and further, without needing arranging an access hole or thelike for a worker to perform welding on the side plates 47 b inside thestructure.

[0115] Needless to say, the present invention can also be applied to thewelding between the upper plate 74 a and the side plates 74 b. When awelding torch or the like can be inserted between both of the sideplates 74 b, 74 b and a sufficient space is available there, it ispossible, as in the conventional art, to bring the side plates 74 b, 74b, on outer sides of which grooves are arranged, into abutment againstthe upper plate 74 a to perform fillet welding to the grooves from theoutside and then to perform fillet welding from the back sides of thegrooves.

[0116] On the other hand, the arm 76 is constructed of a square U-shapedmember 76 a, which forms a box-shaped structure and has been formed bypressing or the like, and a plate-shaped member 76 c which makes up theremaining one side. Upon forming the arm 76 by the welding method of thepresent invention, the member 76 a with root faces formed thereon andalso with single groove faces formed on the outer sides of the rootfaces is brought into abutment against the lower plate 76 c having aflat surface, and by using the above-mentioned welding method shown inFIG. 1, welding is performed from the side of the grooves. By thiswelding, beads 76 g can be formed in grooves between the single groovefaces of the member 76 a and the flat surface of the lower plate 76 c,and further, beads B76 in the form of fillet welds can also be depositedbetween the surfaces on the back side of the grooves of the member 76 aand the flat surface of the lower plate 76 c.

[0117] As has been described above, even in a welded structure such asthat illustrated in FIG. 14, a box-shaped, welded structure welded alsoon the inner side of the structure can be fabricated by welding workonly from the outside of the structure without needing arranging backingstrips or arranging beads for the prevention of a burn-through, apunch-through or the like and further, without needing arranging anaccess hole or the like for a worker to perform welding on the sideportions of the member 76 a inside the structure.

[0118]FIG. 15 is a cross-sectional view depicting the butt-welded jointof the upper plates themselves of different thicknesses in the boom ofthe hydraulic excavator, said butt-welded joint being designated at signXV in FIG. 12. It is the butt-welded joint of the thick plate member 74a′ and the thin plate member 74 a″, both of which make up the upperplate of the boom 74. Both of the members 74 a′,74 a″ are kept inabutment at end portions thereof such that as viewed in the drawing,their lower surfaces lie in the same plane. The member 74 a″ is providedwith a single groove face 74 k. In this embodiment, the thick platemember 74 a′ is provided with a tilted surface extending toward an areaof its abutment with the thin plate member 74 a″, for example, between aposition T1 and a position T2 so that no extreme change occurs instrength.

[0119] In the welded joint illustrated in FIG. 15, the welding methodillustrated with reference to FIG. 6 in connection with the thirdexample of the first embodiment is applied, and welding is performedfrom the side of the groove 74 k such that an arc is generated at apredetermined inclination relative to the butted end face of the member74 a′. As a result, a bead B74′ in the form of a fillet weld can beformed between the butted end face of the member 74 a′ and the back sideof the member 74 a″, said back side being located on a side opposite thesingle groove face 74 k.

[0120] 5. Fifth Embodiment

[0121]FIG. 16 through FIG. 21 are intended to describe a fifthembodiment of the present invention. In this embodiment, elementsequivalent to the corresponding elements described above are designatedby like reference signs, and overlapping descriptions are omitted.

[0122] 5.1 Overall Construction

[0123]FIG. 16 is a diagram showing the outline construction of a weldingapparatus according to this embodiment. In the drawing, the weldingapparatus is constructed basically of a robot main body 3, a robotcontroller 4, and a first and second base materials 1,2 to be welded.The first base material 1 is provided at an end portion on its abuttedside with a single groove face 1 b which serves to form a single bevelgroove. A portion of the second base material 2, said portion being keptin abutment with the first base material 1, is formed as a flat surface.

[0124] The robot main body 3 is constructed such that it can performrevolution and pitching. The robot main body 3 is provided with aparallel link mechanism 3A, and on the parallel link mechanism 3A, arobot arm 3B is arranged. Further, the robot main body 3 is controlledby the robot controller 4. This robot controller 4 is arranged to outputcommand signals such that the robot arm 3B is driven under control inthe directions of at least three orthogonal axes X, Y, Z shown in thedrawing. The robot controller 4 is provided with an input unit 4A forinputting and storing various preset values and the like for control, aposition control unit 4B for controlling the position and attitude ofthe robot arm 3B, a speed control unit 4C for controlling the weldingspeed, etc. A welding torch 5 is arranged on a free end of the robot arm3B, a welding wire 6 is held in place on the welding torch 5, and thewelding wire 6 is fed by a feeder 8 through the torch 5. Designated atsigns 7A and 7B are cables for feeding welding electric power. The cable7A is connected to the welding wire 6 on the side of the feeder 8 whichthe robot is provided with, while the cable 7B is connected to the basematerial 1 or 2 to be welded. The cables 7A, 7B are connected to awelding power source 9, thereby connecting the welding wire 6 and thebase material 1 or 2 to the welding power source 9. A voltage detector10 and current detector 11 are attached to the cable 7A, so that thevoltage of a welding current to be fed from the welding power source 9to the welding wire 6 is detected by the voltage detector 10 and thewelding current to be fed from the welding power source 9 to the weldingwire 6 is detected by the current detector 11. Incidentally, the weldingvoltage and welding current so detected are inputted to a powerdiagnosis unit 23.

[0125] At a preceding stage of the welding power source 9, an outputcontrol device 22 is arranged. At a preceding stage of the outputcontrol device 22, a voltage control device 13 and current controldevice 15 are arranged in parallel with each other. At a preceding stageof the voltage control device 13, an average voltage setting device 12is arranged, and at a preceding stage of the current control device 15,an average current setting device 14 is arranged. The average voltagesetting device 12 sets an average level of an voltage to be fed to thewelding wire 6. The voltage control device 13 compares the averagevoltage level set at the average voltage setting device 12 with anactual average voltage inputted via the below-described power diagnosisunit 23 on the basis of an actual voltage detected by the voltagedetector 10, and outputs a voltage control signal. The average currentsetting device 14 sets an average level of a current to be fed to thewelding wire 6, and the current control device 15 compares the averagecurrent level set at the average current setting device 14 with anactual average current inputted via the below-described power diagnosisunit 23 on the basis of an actual current detected by the currentdetector 11, and outputs a current control signal. Inputted to theoutput control device 22 are the voltage control signal from the voltagecontrol device 13 and the current control signal from the currentcontrol device 15. Based on these control signals, the output controldevice outputs a control signal to control electric power to be fed fromthe welding power source 9.

[0126] The power diagnosis unit 23, to which detection outputs areinputted from the voltage detector 10 and current detector 11, iscomposed of an input unit 23A, a comparison unit 23B and an output unit23C. The input unit 23A sets a first preset voltage level higher involtage than the average voltage level set by the average voltagesetting device 12, a second preset voltage level lower in voltage thanthe average voltage level set by the average voltage setting device 12,and a preset current level higher in current than the average currentlevel set by the average current setting device 14. The comparison unit23B compares an actual voltage inputted from the voltage detector 10with the above-described, first and second preset voltage levels, andoutputs a position control signal, which is to be described subsequentlyherein, to the robot controller 4. Further, the comparison unit comparesan actual current inputted from the current detector 11 with theabove-described, first preset current level or second preset voltagelevel, and outputs a position control signal to the robot controller 4.The output unit 23C outputs the detected actual voltage and actualcurrent from the voltage detector 10 and current detector 11 to thevoltage control unit 13 and current control unit 15, respectively. Inaddition, the welding apparatus is equipped with a computer or the like,which systematically controls the individual controller and controldevices, the robot and the like.

[0127] 5.2 Operation

[0128] Welding operation which makes use of the welding apparatusconstructed as described above is performed as will be describedhereinafter.

[0129] In FIG. 16, the welding torch 5 is mounted on the robot arm 3B.The welding wire 6 is held in place on the welding torch 5, extends overa predetermined length from the free end of the welding torch 5, and isfed by the welding wire feeder 8. As illustrated in FIG. 1, the weldingwire 6 is inserted into a groove formed as a result of contact between afirst base material 1, on which a single groove face 1 b is formed, anda second base material 2 having a flat surface 2 a. At this time, thewelding wire 6 is arranged such that it is tilted at a predeterminedinclination ä relative to the flat surface 2 and is directed toward apredetermined, aimed point P₂. Such a position control is performed bythe input unit 4A and position control unit 4B in the robot controller4. Further, a moving path and welding speed are inputted from the inputunit 4A to the speed control unit 4C, and based on them, the weldingwire 6 is caused to move in a direction perpendicular to the drawingsheet. From the input unit 23A in the power diagnosis unit 23, first andsecond, preset voltage levels and current levels are inputted inadvance, and they are stored in the comparison unit 23B.

[0130] With the welding apparatus preset as described above, arc weldingof a first layer is initiated. As soon as the welding begins, thevoltage detector 10 and current detector 11 detect an actual voltage andactual current in the cable 7A, respectively. If the welding wire 6 iscaused to move too much, for example, toward the side of the groove ofthe first base material 1 during the welding due to vibrations of therobot or positional displacements of the first and second base materials1,2, an end portion of the first base material 1 of smaller thermalcapacity, said end portion being located on the side of the singlegroove face 1 b, is caused to burn through, thereby making it impossibleto form a bead. When the end portion of the first base material 1, saidend portion being located on the side of the single groove face 1 b,undergoes a burn-through, the arc strikes through the first basematerial 1 to its back side so that the actual voltage becomes higher.The comparison unit 23B in the power diagnosis unit 23 compares thefirst and second preset voltage levels, which were inputted in advance,with the actual voltage and, if the actual voltage exceeds the firstpreset voltage level, outputs a position control signal to the positioncontrol unit 4B in the robot 3 such that the welding wire 6 is caused tomove toward the second base material 2. As a result, the welding wire 6is corrected in position such that it moves from the side of the firstbase material 1 toward the second base material 2.

[0131] If the relative positional relationship between the welding wire6 and the first and second base materials 1,2 is changed such that thewelding wire 6 is conversely caused to move from the preset positiontoward the side of the second base material 2, a thermal input into thefirst base material 1 is reduced so that a melt is no longer forced outto the back side of the groove and the melt accumulates on the nearside. This leads to a decrease in the extension of the welding wire 6,resulting in occurrence of a phenomenon that the actual voltage becomeslower and the actual current becomes higher. The comparison unit 23Bthen compares the preset voltage levels, which were inputted beforehand,with the actual voltage and, if the actual voltage falls below thesecond preset voltage level, outputs a position control signal to theposition control unit 4B such that the welding wire 6 is caused to movetoward the groove. As a result, the welding wire 6 is corrected inposition to move toward the first base material 1. As an alternative,because the actual current has been detected by the current detector 11,the comparison unit 23B compares the preset current level, which wasinputted in advance, with the actual current and, if the actual currentis higher than the preset current level, outputs a position controlsignal to the position control unit 4B such that the welding wire 6 iscaused to move toward the first base material 1. The welding wire 6 maybe corrected in position to move toward the first base material 1 inthis manner.

[0132] The output unit 23C in the power diagnosis unit 23, on the otherhand, outputs the actual voltage and actual current, which have beendetected by the voltage detector 10 and current detector 11, to thevoltage control device 13 and current control device 15, respectively.At the voltage control device 13, the preset average voltage level andthe actual voltage are compared with each other to output a voltagecontrol command signal to the output control device 22 such that theactual voltage becomes equal to the preset average voltage level. Thecurrent control device 15, on the other hand, compares the presetaverage current level and the actual current with each other, andoutputs a current control command signal to the output control device 22such that the actual current becomes equal to the preset average currentlevel. Based on the voltage control command signal and current controlcommand signal so inputted, the output control device 22 outputs controlsignals to the welding power source 9 to adjust the voltage and currentto be fed from the welding power source 9 so that the electric power tobe fed is adjusted to the preset values.

[0133] In the welding apparatus according to this embodiment, welding isperformed while correcting the positional relationship between thewelding wire 6 and the first and second base materials 1,2 by moving thewelding wire to the preset position and also while correcting theaverage voltage and average current to the present values, as describedabove. By conducting welding only from the side of the groove, stablewelding can therefore be performed without leaving any undeposited partin the groove while depositing a bead in the form of a fillet weld onthe back side 1 c of the first base material 1, which is located on theback side of the groove, and the flat surface 2 a of the second basematerial 2.

[0134] 5.3 Welding Under Weaving

[0135] With the welding apparatus according to this embodiment, weldingcan also be performed while causing the welding wire 6 to weave. Weavingis effected as explained above with reference to FIG. 10, namely, bycausing the welding torch 5 and welding wire 6 to oscillate with apredetermined oscillation width as indicated by the arrow 50 or arrow 60while keeping their attitudes tilted at a predetermined inclination a oras explained above with reference to FIG. 11, namely, by causing thewelding wire 6 to oscillate and swing tracing a circular arc about theinclination ä as a center. These weaving operations are effected bycontrol signals transmitted from the position control unit 4B to therobot on the basis of the input signals inputted beforehand from theinput unit 4A of the robot 3.

[0136] Examples of the weaving are shown in FIG. 17 and FIG. 18. FIG. 17is a schematic view illustrating an example of weaving operation of awelding wire by the welding apparatus shown in FIG. 16, and FIG. 18 is adiagram showing a relationship between the weaving position of thewelding wire and the welding voltage generated in the welding wire underwelding conditions which minimize the forcing-out of a melt to the backside of a groove by the welding apparatus shown in FIG. 16.

[0137] The characteristics indicated by a solid line in FIG. 18 indicatethe relationship between the weaving position (time) and the weldingvoltage generated in the welding wire 6 in the case that, as illustratedin FIG. 17, a center of oscillation width of weaving when the weldingtorch 5 and welding wire 6 were tilted at the predetermined inclinationä relative to the flat surface 2 a of the second base material 2 andwere directed toward the aimed point P₂ was designated by L₀ and thewelding torch 5 and welding wire 6 were caused to oscillate from thecenter L₀ to the position L₁ on the side of the first base material 1and in the case that the welding torch 5 and welding wire 6 were causedto oscillate from the center L₀ to a position L₂ on the side of thesecond base material 2. To simplify the description, the diagramrepresents a case in which the predetermined inclination a was set at ½of the angle of the single bevel groove on the first base material 1 andthe weaving center L₀ shown in FIG. 17 was set at a position directedtoward the aimed point P₂.

[0138] The characteristics indicated by a dashed curve in FIG. 18, onthe other hand, indicate the relationship between the weaving position(time) and the welding voltage generated in the welding wire 6 in thecase that the inclination a of the welding wire 6 was set at ½ of theangle of the single bevel groove as in the above-mentioned cases, theaimed point of the welding wire 6 was set at the position of contact P₁where the end portion of the single groove face 1 b is in contact withthe flat surface 2 a of the second base material 2, the weaving centerof the welding wire 6 was set at L_(0′) (not shown; this will applyequally hereinafter), and the welding wire 6 was caused to oscillatefrom the center L_(0′) to a position L_(1′) on the side of the firstbase material 1 and in the case that the welding wire 6 was caused tooscillate to a position L_(2′) of the second base material 2(L_(1′)=L_(2′)). In those cases, however, the welding conditions wereselected such that the forcing-out of a melt to the back side of thegroove by an arc would be minimized.

[0139] Now assume that weaving is performed with the aimed point set atthe contact position P₁. When the weaving center L_(0′) is directedtoward the contact position P₁, the welding wire 6 located at the centerL_(0′) defines a relatively longest distance from the tip of the weldingwire 6 to the groove and therefore, the welding voltage takes thehighest value as indicated by the dashed line in FIG. 18. If the weldingwire 6 is caused to oscillate toward the first base material 1subsequently, the distance between the tip of the welding wire 6 and thefirst base material 1 becomes gradually shorter so that, as indicated bythe dashed line in FIG. 18, the voltage gradually drops and becomeslowest at the position L_(1′) where the welding wire 6 is locatedclosest to the first base material 1. When the welding wire then beginsto return toward the second base material 2, the voltage, as indicatedby the dashed line in FIG. 18, gradually rises back and becomes highestagain at the center L_(0′) of oscillation, and then becomes lowest atthe position L_(2′) where the welding wire is located closest to thesecond base material 2. This is repeated.

[0140] A description will next be made of a weaving operation when, asillustrated in FIG. 17, the aimed point of the welding wire 6 was set atP₂, the position where the inclination angle was ä was set at theweaving center L₀ and the welding wire 6 was caused to oscillate fromthe center L₀ to the position L₁ on the side of the first base material1, and also of a weaving operation when the welding wire 6 was caused tooscillate to the position L₂ on the side of the second base material 2.

[0141] When the welding wire 6 is located at the weaving center L₀, thedistance between the tip of the welding wire 6 and the groove is not themaximum. As shown in FIG. 18, the voltage in the above case, therefore,takes a value which corresponds to the distance. As the welding wirethen moves toward the first base material 1, the voltage, as indicatedby the solid curve in FIG. 18, gradually rises, and reaches the maximumvoltage when the welding wire 6 has directed toward the contact positionP₁. The voltage then gradually drops up to the position L₁, and when thewelding wire then begins to return toward the second base material 2,the voltage reaches the maximum voltage again at the contact positionP₁. Because the welding wire thereafter gradually moves closer to thesecond base material 2, the voltage drops to the lowest voltage at theposition L₂. This is repeated.

[0142] The relationship between the position of the welding wire 6 andthe voltage can be determined either empirically or by calculation inaccordance with preset conditions such as the groove shape, the aimedpoint and the inclination. These values are set beforehand from theinput unit 23A to the comparison unit 23B in the power diagnosis unit23.

[0143] Here, it is to be noted that positional control on the weldingtorch 5 for the above-described weaving is set beforehand from the inputunit 4A to the position control unit 4B in the robot controller 4.Further, welding voltages are detected by the voltage detector 10, andbased on the detection values, actual voltages and an actual averagevoltage are determined by the comparison unit 23B. By a comparisonbetween an actual voltage and a preset average voltage level, thecomparison unit 23B determines whether or not an actual aimed point iscorrect. When an offset has occurred, a position control signal isoutputted to the position control unit 4B of the robot controller 4, andthe welding torch is controlled such that the aimed point is broughtinto registration with the preset position.

[0144] Further, an actual average voltage V₀ is determined by thecomparison unit 23B of the power diagnosis unit 23 on the basis ofdetection values of the voltage detector 10. The value V₀ is outputtedto the voltage control device 13 via the output unit 23C, and from thevoltage control device 13, a command signal is outputted to the outputcontrol device 22 such that the actual voltage becomes equal to thepreset average voltage level as described above. The voltage is adjustedaccordingly.

[0145] In the above-described case, the welding conditions were selectedsuch that the forcing-out of a melt to the back side of a groove by anarc would become minimum. When the welding conditions are selected suchthat a bead will be formed to the maximum on the back side of a groove,on the other hand, and further, when the weaving center L₀ shown in FIG.17 is located on the contact position P₁ (in other words, the weavingcenter is located at the above-mentioned L_(0′) not shown in thedrawing), a welding operation is performed as will be describedhereinafter.

[0146]FIG. 19 is a diagram showing a relationship between the weavingposition of the welding wire and the welding voltage generated in thewelding wire under such welding conditions that maximize the forcing-outof a melt to the back side of a groove by the welding apparatus shown inFIG. 16.

[0147] In this case, in a range that the welding wire 6 moves from theabove-mentioned, unillustrated center L_(0′) toward the second basematerial 2, reaches the position L_(2′) and then returns to the centerL_(0′), the voltage waveform shows the same locus as the voltageindicated by the above-described, dashed curve in FIG. 18. When thewelding wire 6 moves past the center L_(0′) and is oscillated toward thefirst base material 1, however, an excessively large quantity of heatenters an end portion of the first base material 1 of small thermalcapacity, said end portion being located on the side of the singlegroove face 1 b, at a certain position beyond the center L_(0′) so thatpunching-through of an arc to the back side of the groove takes place.An arc is then formed between the tip of the welding wire 6 and thesecond base material 2, resulting in a sudden increase in the distancetherebetween.

[0148] As a result, the voltage which has varied following the samelocus as the dashed curve in FIG. 18 leaves the locus and increasessuddenly. The voltage then rises further little by little because thedistance between the tip of the welding wire 6 and the second basematerial 2 gradually widens until the position L_(1′) is reached. Whenthe welding wire begins to return from the position L_(1′) the voltagealso begins to drop gradually. Concurrently with elimination of apunching-through as a result of a decrease in the input of heat to theend portion of the first base material 1, the voltage returns onto theabove-mentioned locus of the dashed curve in FIG. 18, and on and alongthe locus, eventually reaches the position L_(0′).

[0149] As has been described above, the time interval during which thevoltage suddenly rises when the welding wire 6 is located on the side ofthe first base material 1 relative to the position L_(0′) has acorrelation with the amount of a bead to be formed on the back side ofthe groove. Like the above-mentioned, dashed curve in FIG. 18, theformation of a bead is minimized if the time interval during which thevoltage suddenly rises does not exist. It has also been found that, ifthis time interval is too long, on the other hand, punching-through ofthe arc takes place excessively to separate the first base material 1and the bead B4 from each other and hence, no good welding results areavailable.

[0150] It is, therefore, necessary to control the duration of this timeinterval in accordance with a desired amount of a bead to be formed.Especially when there is a time interval during which the voltage risessuddenly, it is necessary to control its duration.

[0151]FIG. 20 and FIG. 21 are flow charts illustrating a controlprocedure for the weaving position of the welding wire in the weldingapparatus shown in FIG. 16. This control is performed by the robotcontroller 4. In this control, the voltage is firstly monitored to checkwhether or not the voltage is at the maximum point V₁. Upon detection ofthe maximum point V₁ (a voltage corresponding to the position P₁), it isdetermined from three-dimensional coordinate data of the robot main body3 and the direction of an operation of the welding torch 5 whether thecurrent position of the welding torch 5 is on the side of the first basematerial 1 or on the side of the second base material 2 relative to theweaving center (step S2). The routine returns to step S1 if the currentposition of the welding torch is on the side of the second base material2, or awaits an elapse of a first predetermined time Δt1 if the currentposition of the welding torch is on the side of the groove (step S3).When the first predetermined time Δt1 has elapsed, it is determinedwhether or not the voltage has arisen above a predetermined threshold V2(step S4). If “NO”, it is determined whether or not a secondpredetermined time Δt2 has elapsed (step S5). If “YES”, step S13 isperformed as will be described subsequently herein. If the secondpredetermined time Δt2 is not determined to have elapsed, the routinereturns to step S4. If the voltage is determined to have arisen abovethe threshold V2 in step S4, a current time t3 is stored in the inputunit 4A (step S6). Subsequent to the storage of the time t3, it isdetermined whether or not the voltage has fallen below the threshold V2(step S7). If “NO”, it is determined whether or not the secondpredetermined time Δt2 has elapsed (step S8). If “YES”, step S12 isperformed as will be described subsequently herein. If the secondpredetermined time Δt2 is not determined to have elapsed, the routinereturns to step S7. If the voltage is determined to have fallen belowthe threshold V2 in step S7, a current time t4 is stored (step S9).

[0152] Next, the routine awaits a lapse of the second predetermined timeΔt2 (step S1). If the second predetermined time Δt2 is determined tohave elapsed, the difference between the time t4 and the time t3 (t4−t3)is compared with a target time Δt0 that permits adequate melting of theend portion of the first base material 1 without being punched through(step S11). If the former is longer than the latter, the amount ofpunching-through is excessive so that the weaving center is correctedtoward the second base material 2 (step S12). If they are equal to eachother, the weaving center is left unchanged. If the former is shorterthan the latter, the amount of punching-through is insufficient so thatthe weaving center is corrected toward the first base material 1 (stepS13).

[0153] If the second predetermined time Δt2 is determined to haveelapsed without the voltage arising above the threshold V2 in step S5,on the other hand, the amount of punching-through is insufficient sothat the weaving center is corrected toward the first base material 1(step S13). If the second predetermined time Δt2 is determined to haveelapsed without the voltage falling below the threshold V2 in step S8,on the other hand, the amount of punching-through is excessive so thatthe weaving center is corrected toward the second base material 2 (stepS12). As an alternative, these cases may be taken as an indication of apoor bead, and a command may be sent to the robot controller 4 to stopthe welding.

[0154] The amount of the correction of the weaving center in step S12 orstep S13 is suitably changed depending on the magnitude of thedifference in step 11 and also depending on whether the correction isperformed after step S5 or after step S8, and a command is outputted tothe robot controller 4. In addition to the correction of the weavingcenter, welding conditions such as welding current and speed may also bechanged.

[0155] Further, the threshold V2 and target time Δt0 and the first andsecond predetermined times Δt1,Δt2 are changed depending on the weldingconditions, the groove shape, the target amount of a bead to be formed,etc.

[0156] As has been described above, the weaving center can beautomatically corrected based on changes in actual voltage as monitoredwhen the welding wire 6 is located on the side of the first basematerial 1. Even if the first and second base materials 1,2 are causedto warp upward or downward during welding such that the aimed pointbecomes offset from the preset point, the welding can be still continuedwhile correcting the offset. A bead in the form of a fillet weld can,therefore, be caused to deposit efficiently with higher accuracy on theopposite side of the groove by simply performing arc welding from theside of the groove.

[0157] If the groove shifts or deforms in the direction of its distanceto the tip of the welding wire 6, the welding wire is also controlled tomove along a weld line.

[0158] In such a case, time is clocked from a return of the welding wire6 to the contact position P₁ subsequent to its oscillation toward thegroove of the first base material 1 by weaving until a return of thewelding wire 6 to the contact position P₁ subsequent to its oscillationtoward the second base material 2 (see FIG. 19 described above). Duringthis time, actual voltages fed to the welding wire 6 are measured, andan actual average voltage during a single reciprocation of weaving fromthe contact point P₁ as the base point toward and from the second basematerial 2 is computed. Now assume that in FIG. 19, V₀ is a presetaverage voltage level. When the actual average voltage rises above apredetermined upper limit threshold V_(OU), the distance between thewelding wire 6 and the welded portion in the groove is taken to begreater than a predetermined, preset value and the welding wire 6 ismoved toward the groove (in FIG. 17, in the horizontal direction towardthe contact position P₁). When the above-described actual averagevoltage conversely falls below a lower limit threshold V_(OL), thedistance between the welding wire 6 and the groove is taken to besmaller than the predetermined, preset value and the welding wire 6 ismoved away from the groove (from the side of the contact position P₁ inFIG. 17) in the horizontal direction.

[0159] When weaving is performed as described above, stable leg lengthsas much as 7 to 5 mm in the vertical direction (the solid line whichconnects square dots) and 4 to 3 mm in the horizontal direction (thesolid line which connects triangle dots) can be obtained in a wide rangethat the distance between the contact position P₁ and the aimed point P₂ranges from 0.5 mm to 5 mm, as illustrated in FIG. 24. In other words,such weaving makes it possible to provide the offset of the aimed pointwith a greater tolerance.

[0160] In the foregoing, the description was made about the case inwhich variations in voltage were monitored. As an alternative,variations in current may be monitored. In such a case, increases ordecreases take place in a form reversed to those in the case of voltage.As the welding current, a current of pulse waveform, a sunisoidalcurrent or the like, which has high directivity, can be used instead ofa direct current, although use of a high-directivity current such as acurrent of pulse waveform is preferred.

[0161] 6. Sixth Embodiment

[0162]FIG. 22 is the diagram showing the construction of the weldingapparatus according to the sixth embodiment of the present invention. Inthis embodiment, elements equivalent to the corresponding elements inthe above-described fifth embodiment shown in FIG. 16 are designated bylike reference signs, and overlapping descriptions are omitted.

[0163] The welding apparatus according to the sixth embodiment isconstructed such that a pulse current is superimposed on a weldingcurrent. To the welding apparatus depicted in FIG. 16, a pulse waveformcontrol device 19 is added in parallel with the voltage control device13 and current control device 15 at a stage preceding an output controldevice 22′. At a stage preceding the pulse waveform control device 19, apeak voltage setting device 16 for setting a peak voltage for the pulsecurrent, a base voltage setting device 17 for setting a base voltage forthe pulse current and a pulse duration setting device 18 for setting apulse duration for the pulse current are arranged in parallel with oneanother, and the real time of the pulse duration is inputted from apower diagnosis unit 23′. To the pulse waveform control device 19, thepeak voltage, base voltage and pulse duration are inputted from the peakvoltage setting device 16, base voltage setting device 17 and pulseduration setting device 18, respectively. Based on these inputs, adesired pulse waveform is produced as a basis.

[0164] Based on control signals inputted from the voltage control device13, current control device 15 and pulse waveform control device 19, theoutput control device 22′ controls welding electric power to beoutputted from the welding power source 9. The power diagnosis unit 23′is composed of an input unit 23A′, a comparison unit 23B′, an outputunit 23C, and the like. The input unit 23A′ sets a first preset voltagehigher than the average voltage set by the average voltage settingdevice 12, a second preset voltage lower than the average voltage set bythe average voltage setting device 12, and a preset current higher thanthe average current set by the average current setting device 14. Thecomparison unit 23B′ compares an actual voltage, which is inputted fromthe voltage detector 10, with the first and second present voltages andoutputs a positional control signal, which will be describedsubsequently herein, to the robot controller 4, and also compares anactual current, which is inputted from the current detector 11, with thepreset current and outputs a positional control signal to the robotcontroller 4. The comparison unit 23B′ is also equipped with a functionto determine, in addition to an actual average voltage and an actualaverage current, the actual peak current (or voltage) of the pulsecurrent, the actual base current (or voltage) of the pulse current andthe real time (width) of each pulse on the basis of detection valuesdetected by the voltage detector 10 and current detector 11,respectively. From the output unit 23C′, the actual average voltage andthe actual average current are outputted to the voltage control device13 and the current control device 15, respectively, while the actualpeak current, the actual base current and the real time of the pulseduration are outputted to the pulse waveform control device 19. Like thefifth embodiment, the welding apparatus is equipped with a computer orthe like, which controls all of the above-described individual controldevices, robot and the like.

[0165] Welding which makes use of the welding apparatus constructed asdescribed above is performed as will be described hereinafter.

[0166] Reference is had to FIG. 22. When from the peak voltage settingdevice 16, base voltage setting device 17 and pulse duration settingdevice 18, their respective preset values are inputted to the pulsewaveform control device 19, a pulse waveform is produced at the pulsewaveform control device 19. Its waveform control signal is outputted tothe output control device 22′. Based on an average voltage controlsignal from the voltage control device 13, an average current controlsignal from the current control device 15 and the pulse waveform controlsignal from the pulse waveform control device 19, the output controldevice 22′ controls the welding power source 9 such that electric powerof the desired waveform is supplied from the welding power source 9.Depending on the characteristics of the power source 9 employed for thewelding, the pulse waveform is set based on one or both of current andvoltage.

[0167]FIG. 23 is, as already referred to in brief, the waveform diagramof the pulse current for the welding apparatus according to the fifthembodiment shown in FIG. 22.

[0168]FIG. 23 shows a peak current A_(p), a base current A_(b), and apulse width, namely, pulse duration T_(p). A_(a) indicates an averagecurrent. This average current A_(a) is set by the above-described,average current setting device 14. Based on these preset values A_(p),A_(b), T_(p) and A_(a), the width, namely, time T_(b) of the basecurrent of pulses is determined by the above-described output controldevice 22′, and a waveform in the form of a direct current with a pulsewaveform superimposed thereon is supplied as an output control signal tothe welding power source 9.

[0169] More specifically, with a waveform indicated by a solid line inFIG. 23, the peak current A_(p), base current A_(b) and pulse durationT_(p) are given, for example, as 400 A, 200 A and {fraction (1/50)} sec,respectively. Further, a value determined as the width, namely, durationT_(b) of the base current on the basis of an average current A_(a) of300 A given from the average current setting device 14 (in this case,T_(b)=T_(p)={fraction (1/50)} sec) is indicated.

[0170] When a pulse current obtained by super imposing a pulse-likecurrent on a direct current is used as a welding current as describedabove, the directivity of an arc is enhanced compared with use of acurrent consisting of a direct current alone, thereby making it possibleto produce the arc more accurately at a point aimed by the welding wire6 and hence to perform more stable arc welding of a groove.

[0171] On the other hand, a waveform indicated by a double dashed linein FIG. 23 is a pulse waveform obtained, for example, when the peakcurrent was raised further by 100 A to A_(p′) (=500 A), the pulseduration was shortened to ½ of that in the case of the solid line, andthe average current A_(a) and base current A_(b) were set at 300 A and200 A, respectively, as in the case of the solid line.

[0172] In this case, the base current A_(b) (=200 A) and average currentA_(a) (=300 A) were set at the same levels as in the case of thewaveform indicated by the solid line. In the case of the waveformindicated by the solid line, the cycle of pulses is (T_(p)+T_(b)). Inthe case of the waveform indicated by the double dashed line, on theother hand, the cycle is therefore shortened to ((T_(p)×½)+T_(b)).Expressing this in terms of duty, the duty in the case of the waveformindicated by the solid line is expressed by:

T _(p)/(T _(p) +T _(b))=({fraction (1/50)})/(1/50+{fraction (1/50)})=½

[0173] whereas the duty in the case of the waveform indicated by thedouble dashed line is expressed by:

T _(p′)/(T _(p′) +T _(b′))=({fraction (1/50)}×½)/(({fraction(1/50)}×½)+{fraction (1/50)})=⅓

[0174] A waveform indicated by a dashed line in FIG. 23 is a pulsewaveform obtained when relative to the waveform indicated by the solidline, the peak current A_(p), base current A_(b) and average currentA_(a) were left unchanged at 400 A, 200 A and 300 A, respectively, as inthe case of the solid line and the pulse duration T_(p″) was shortenedto ½ of that in the case of the waveform indicated by the solid line. Inthis case, the base current width T_(b″) becomes equal to T_(p″), andthe duty is ½ as in the case of the waveform indicated by the solidline.

[0175] In FIG. 23, some examples of pulse waveforms are shown asdescribed above. An arc is provided with improved directivity when theduty of a pulse waveform is rendered smaller. However, the averagecurrent A_(a), the peak current A_(p) and base current A_(b) of pulses,and the like in welding are correlated to various welding conditions andthe like. From the standpoints of the transfer of globules by a weldingarc, the stability of the arc, and the like, practical values of theaverage current A_(a), base current A_(b) and peak current A_(p) in thisembodiment can be around 300 A, not higher than 300 A (including anegative ampere), and 350 to 500 A or so, respectively.

[0176] As has been described above, the welding apparatus according tothis embodiment, in addition to the control in the welding apparatusaccording to the fifth embodiment described above with reference to FIG.16, uses a pulse current as a welding current and controls the pulsecurrent such that the pulse current becomes equal to a preset value.This has made it possible to increase the directivity of an arc and tocontrol the arc to the aimed point more accurately. As a consequence,stable welding can be performed on the groove without leaving any partundeposited by simply welding from the side of the groove while allowinga bead to deposit in the form of a fillet weld on the back side 1 c ofthe groove.

[0177] Although not illustrated in any drawing, the peak voltage settingdevice 16, base voltage setting device 17, pulse duration setting device18 and pulse waveform control device 19 in the welding apparatusaccording to this embodiment can be replaced by a sunisoidal currentsetting device. Because a sunisoidal current can be used as a weldingcurrent in this case, an arc can be provided with improved directivitysimilarly to the welding apparatus according to the sixth embodiment. Asa result, similar welding can be performed as the welding apparatusaccording to the sixth embodiment.

[0178] As has been described above, the embodiments of the weldingmethod according to the present invention and the embodiments of thewelding apparatus according to the present invention have made itpossible to perform the formation of an auxiliary bead without using anybacking material or the like by one-side welding to the area of contactbetween the first base material 1 and the second base material 2 where agroove such as a single bevel groove or a single J groove is formed by aone-side groove wall, although such one-side welding has heretofore beeninfeasible unless a backing material or a bead, which does notcontribute to the joining, is additionally applied. As a consequence, itis possible to significantly reduce the fabrication cost of abox-shaped, welded structure which has required a substantial time forwelding work inside the structure.

[0179] In a structure inside which welding has heretofore beenimpossible due to a difficulty in inserting the welding torch 5, a leglength reaches both of the side of the groove and the back side of thegroove, thereby bringing about significant advantageous effects in thatthe resulting joint can be provided with substantially improved strengthand the strength of the joint can be retained without unnecessarilyincreasing the thicknesses of two base materials to be brought intoabutment.

[0180] In connection with a butt-welded joint, the present inventionrequires welding work only from one side even if the butt-welded jointis of such a welded structure that has heretofore required to alsoperform welding on the back side of the groove. The present invention,therefore, makes it possible to efficiently decrease the number ofwelding steps. Owing to these advantageous effects, it is also possibleto achieve diversification and to improve efficiency as to the designingof a welded structure.

INDUSTRIAL APPLICABILITY

[0181] As has been described above, the welding method according to thepresent invention makes it possible to achieve welding in a groove andalso to stably deposit a bead in the form of a fillet weld on the backside of the groove owing to a difference in thermal capacity between afirst base material provided with the groove face and a second basematerial having a flat surface and fusion of a welding wire. Describedspecifically, a bead in the form of a fillet weld can also be formed bycausing a greater portion of an arc to generate on the side of thesecond material the thermal capacity of which is greater; causing thefirst base material, the thermal capacity of which is smaller, to fuseto such an extent that an end portion of the first base material, saidend portion being located on the side of a single groove face, is notpunched through; and forcing out a melt of the welding wire to the backside of the groove while filling up the groove owing to melting of thewelding wire. As a result, no work is needed for the arrangement of anauxiliary bead so that the workability can be improved significantly.

[0182] In addition, according to welded joints and welded structures ofthe present invention, the fabrication cost is low and further, thefabrication time can be shortened. It is, therefore, possible to improvethe productivity of products in which such welded joints and/or weldedstructures are incorporated.

[0183] According to the welding apparatus of the present invention, theposition of the welding wire can be controlled with high accuracy. Itis, therefore, possible to provide welded joints, structures of closedcross-sections and the like, each of which has been fabricated such thatone or more beads in the form of fillet weld or welds are depositedinside by arranging a single groove face at at least one location on anouter side of a base material and then performing welding to the groovefrom the outside. Accordingly, their productivity can be improvedsubstantially.

[0184] Moreover, the tolerance of an offset from an aimed point can beenlarged by weaving the welding wire. As a result, welding can beperformed with high accuracy and high efficiency, thereby significantlyimproving the welding workability.

1. A welding method for butt welding a first base material and a secondbase material with each other, which comprises: bringing said first basematerial, on an end portion of which a root face and a single grooveface have been formed, into contact at said root face thereof with aflat surface of said second base material, arranging a welding wire toface on a groove formed by said contact between said first base materialand said second base material, causing said first base material and saidsecond material to fuse at an area of contact therebetween by an arcfrom said welding wire, and forcing a melt, which has been formed bysaid fusion, out to a back side of said groove to form a bead on saidback side of said groove.
 2. A welding method for butt welding a firstbase material and a second base material with each other, whichcomprises: positioning a center axis of an arc from a welding wire at anaimed point, which is located either at a position of contact between asingle groove face formed on said first base material and a flat surfaceof said second base material or at a position adjacent and not fartherthan said position of contact, obliquely irradiating said arc at apredetermined inclination onto said flat surface of said second basematerial in a direction passing through said aimed point such that saidfirst base material and said second base material are caused to fuse atan area of contact therebetween, and forcing a melt, which has beenformed by said fusion, out to a back side of said groove to form a beadon said back side of said groove.
 3. A welding method according to claim2, wherein said welding wire is caused to weave with a predeterminedoscillation width within said groove while maintaining saidpredetermined inclination.
 4. A welding method according to claim 2,wherein said welding wire is caused to weave with a predeterminedoscillation width with a center of weaving set at a predetermined pointon said predetermined inclination within said groove.
 5. A weldingmethod according to claim 3 or 4, wherein: while said welding wire isweaving, a change in welding voltage or welding current during a timeafter said arc moves past said predetermined position until said arcmoves toward said first base material and returns back to said positionof contact is detected based on a time at which said arc passes throughsaid position of contact, and said welding wire is moved in a directionof a width of said groove on a basis of the thus-detected change suchthat said center of said weaving always remains at an appropriateposition relative to said groove.
 6. A welding method according to claim3 or 4, wherein: while said welding wire is weaving, a change in weldingvoltage or welding current during a time after said arc moves past saidpredetermined position until said arc moves toward said first basematerial and returns back to said position of contact is detected basedon a time at which said arc passes through said position of contact, anda tip of said welding wire is moved toward or away from said position ofcontact on a basis of the thus-detected change such that said center ofweaving always remains at an appropriate position relative to saidgroove.
 7. A welding method according to claim 1 or 2, wherein said beadformed on said back side of said groove is a weld bead in a form of afillet weld.
 8. A welding method according to claim 1 or 2, wherein acurrent for generating said arc is a direct current.
 9. A welding methodaccording to claim 1 or 2, wherein a current for generating said arc isa current obtained by superimposing a direct current and a pulse currenton each other.
 10. A welding method according to claim 1 or 2, wherein acurrent for generating said arc is a sinusoidal current.
 11. A welded“T” joint with a first base material and a second base material buttwelded with each other, wherein: said first base material and saidsecond base material have been joined together at an area of contactbetween a flat surface of said second base material and said first basematerial, on which a root face positioned in contact with said flatsurface and a single groove face are formed, by causing said first basematerial and said second base material to fuse at said area of contactwith an arc from a welding wire positioned facing on a groove formed bysaid single groove face and said flat surface and forcing a melt, whichwas formed by said fusion, out to a back side of said groove to form abead in a form of a fillet weld.
 12. A butt-welded “T” joint of flatplates with a first base material and a second base material butt weldedwith each other, wherein: said first base material and said second basematerial have been joined together at an area of contact between an endsurface of said second base material and said first base material, onwhich a root face positioned in contact with said end surface and asingle groove face are formed, by causing said first base material andsaid second base material to fuse at said area of contact with an arcfrom a welding wire positioned facing on said groove and forcing a melt,which was formed by said fusion, out to a back side opposite to a sideof said groove to form a bead in a form of a fillet weld.
 13. Abox-shaped, welded structure formed by butt welding a plurality of basematerials made of plates, wherein: each two base materials butt weldedwith each other have been joined together at an area of contact betweena flat surface of one of said two base materials and the other basematerial, on which a root face positioned in contact with said flatsurface and a single groove face are formed, by causing said basematerials to fuse at said area of contact with an arc from a weldingwire positioned facing on a groove formed by said single groove face andsaid flat surface and forcing a melt, which was formed by said fusion,out to a back side of said one base material, said back side being on aside opposite to a side of said groove, to form a bead in a form of afillet weld.
 14. A welding apparatus for butt welding a first basematerial and a second base material with each other, comprising: awelding wire to be introduced into a groove formed at an area of contactbetween said first base material and said second material, a controldevice for controlling a position of said welding wire, a welding powersource for feeding welding electric power to said area of contact, andcontrol means for outputting a command to said control device such thatsaid welding wire is moved toward said second base material when anactual voltage fed from said welding power source to said area ofcontact has exceeded a first preset voltage level set in advance andsaid welding wire is moved toward said first base material when anactual current fed from said welding power source to said area ofcontact has exceeded a preset current level set in advance or saidactual voltage has fallen below a second preset voltage level set inadvance.
 15. A welding apparatus for butt welding a first base materialand a second base material with each other, comprising: a welding wireto be introduced into a groove formed at an area of contact between saidfirst base material and said second material, a control device forcontrolling a position of said welding wire, a welding power source forfeeding welding electric power to said area of contact, and controlmeans for outputting a command to said control device such that saidwelding wire is moved toward said second base material when an actualvoltage fed from said welding power source to said area of contact hasexceeded a first preset voltage level set in advance and said weldingwire is moved toward said first base material when an actual current fedfrom said welding power source to said area of contact has exceeded apreset current level set in advance or said actual voltage has fallenbelow a second preset voltage level set in advance and also forcomparing said actual current and voltage with a preset average currentlevel and preset average voltage level set in advance, respectively, andoutputting a current control signal and voltage control signal to saidwelding power source such that these actual current and voltage arebrought into conformity with said preset current and voltage levels setin advance.
 16. A welding apparatus for butt welding a first basematerial and a second base material with each other, comprising: awelding wire, a welding torch for holding said welding wire in place,feeding means for feeding said welding wire to said welding torch, acontrol unit provided with a position control device for positionallycontrolling said welding wire in directions of three orthogonal axes, awelding power source for feeding welding electric power to an area ofcontact between said first base material and said second base material,an average voltage setting device for setting an average voltage levelfor said welding, an average current setting device for setting anaverage current level for said welding, a voltage detector and currentdetector for detecting an actual welding voltage and actual weldingcurrent to form, by said welding wire positioned facing on a grooveformed by a single groove face arranged on said first base material anda flat surface of said second base material, a bead in a form of afillet weld on a back side of said groove, a power diagnosis unit forbeing inputted with said actual voltage detected by said voltagedetector and said actual current detected by said current detector andoutputting a command to said position control device such that saidwelding wire is moved toward said second base material when said actualvoltage has exceeded a first preset voltage level set in advance andsaid welding wire is moved toward said first base material when saidactual current detected by said current detector has exceeded a presetcurrent level set in advance or said actual voltage detected by saidvoltage detector has fallen below a second preset voltage level set inadvance and also for outputting said actual voltage and outputting saidactual current, said current control device for comparing said averagecurrent level inputted from said average current setting device withsaid actual current inputted from said power diagnosis unit andoutputting a current control signal such that this actual current isbrought into correspondence with said preset current level set inadvance, a voltage control device for comparing said average voltagelevel inputted from said average voltage setting device with said actualvoltage inputted from said power diagnosis unit and outputting a voltagecontrol signal such that this actual voltage is brought into conformitywith said preset voltage level set in advance, and said output controldevice for being inputted with said current control signal outputtedfrom said current control device and said voltage control signaloutputted from said voltage control device, and based on said currentcontrol signal and voltage control signal, outputting a power sourcecontrol signal to said welding power source to control an output of saidpower source.
 17. A welding apparatus according to claim 16, whereinsaid welding apparatus is provided with: a peak voltage setting devicefor setting a peak voltage for a pulse voltage, a base voltage settingdevice for setting a base voltage, a pulse duration setting device forsetting a pulse duration, and a pulse waveform control device for beinginputted with said preset peak voltage, preset base voltage level andpreset pulse duration from said peak voltage setting device, basevoltage setting device and pulse duration setting device, respectively,comparing these preset values with an actual peak voltage, actual basevoltage and actual pulse duration, respectively, and outputting awaveform control signal to said output control device, said powerdiagnosis unit comprises means for determining an actual peak voltage,actual base voltage and actual pulse duration on a basis of said actualvoltage detected by said voltage detector and said actual currentdetected by said current detector, and outputting the thus-determinedactual peak voltage, actual base voltage and actual pulse duration tosaid pulse waveform control device, and said output control devicecomprises means for being inputted with said waveform control signaloutputted from said pulse waveform control device and outputting a powersource control signal, which controls an output of said power source, tosaid welding power source on a basis of said voltage control signalinputted from said voltage control device and said current controlsignal inputted from said current control device.
 18. A weldingapparatus according to claim 16, wherein: said pulse waveform controldevice comprises sinusoidal current forming means for converting awelding current into a sinusoidal current, comparing a preset sinusoidalwaveform, which has been set in advance, with an actual sinusoidalwaveform and outputting a waveform control signal to said output controldevice, said power diagnosis unit comprises means for determining anactual current waveform on a basis of said actual voltage detected bysaid voltage detector and said actual current detected by said currentdetector, and outputting the thus-determined actual current waveform tosaid sinusoidal current forming means, and said output control devicecomprises means for outputting a power source control signal, whichcontrols an output of said power source, to said welding power source ona basis of said waveform control signal inputted from said sinusoidalcurrent forming means, said voltage control signal inputted from saidvoltage control device and said current control signal inputted fromsaid current control device.
 19. A welding apparatus for butt welding afirst base material and a second base material with each other,comprising: a welding wire to be introduced through a welding torch intoa groove formed by bringing said first base material, on which a rootface and a single groove face have been formed, into contact with saidsecond material having a flat surface, a control device for controllinga position of said welding wire, a welding power source for feedingwelding electric power to said area of contact, a voltage detector fordetecting an actual voltage to be fed from said welding power source tosaid area of contact, a clock device for detecting a time, at which anarc from said welding wire passes through a position of contact betweenan end portion of said single groove face and said flat surface, andclocking a time after said arc passes through said position of contactuntil said arc moves toward said first base material and returns back tosaid predetermined position, memory means for storing a change inwelding voltage during said clocking time by said voltage detector andsaid clock device, and wire position control means for continuingpreceding weaving control when a time (t₄−t₃)−t₃ being a time at whichan actual voltage begins to exceed a preset voltage level set in advanceand t₄ being a time at which said actual voltage falls below said presetvoltage level, both while clocking a second predetermined time Ät₂subsequent to an elapse of a first predetermined time Ät₁ from a timepoint at which said welding wire passed through said position of contactduring said clocking time—falls within a target time range in whichadequate fusion takes place without punching through said first basematerial, moving a center axis of weaving of said welding wire towardsaid second base material when said time (t₄−t₃) exceeds said targettime, and moving said center axis of weaving toward said first basematerial when said time (t₄−t₃) falls below said target time.
 20. Awelding apparatus for butt welding a first base material and a secondbase material with each other, comprising: a welding wire to beintroduced through a welding torch into a groove formed by bringing saidfirst base material, on which a root face and a single groove face havebeen formed, into contact with said second material having a flatsurface, a control device for controlling a position of said weldingwire, a welding power source for feeding welding electric power to saidarea of contact, a current detector for detecting an actual current tobe fed from said welding power source to said area of contact, a clockdevice for detecting a time, at which an arc from said welding wirepasses through a position of contact between an end portion of saidsingle groove face and said flat surface, and clocking a time after saidarc passes through said position of contact until said arc moves towardsaid first base material and returns back to said predeterminedposition, memory means for storing a change in welding current duringsaid clocking time by said current detector and said clock device, andwire position control means for continuing preceding weaving controlwhen a time (t₄−t₃)−t₃ being a time at which an actual current begins tofall below a preset current level set in advance and t₄ being a time atwhich said actual current exceeds said preset voltage level, both whileclocking a second predetermined time Ät₂ subsequent to an elapse of afirst predetermined time Ät₁ from a time point at which said weldingwire passed through said position of contact during said clockingtime—falls within a target time range in which adequate fusion takesplace without punching through said first base material, moving a centeraxis of weaving of said welding wire toward said second base materialwhen said time (t₄−t₃) exceeds said target time, and moving said centeraxis of weaving toward said first base material when said time (t₄−t₃)falls below said target time.
 21. A welding apparatus for butt welding afirst base material and a second base material with each other,comprising: a welding wire to be introduced through a welding torch intoa groove formed by bringing said first base material, on which a rootface and a single groove face have been formed, into contact with saidsecond material having a flat surface, a control device for controllinga position of said welding wire, a welding power source for feedingwelding electric power to said area of contact, a voltage detector fordetecting an actual voltage to be fed from said welding power source tosaid area of contact, a clock device for detecting a time, at which anarc from said welding wire passes through a position of contact betweenan end portion of said single groove face and said flat surface, andclocking a time after said arc passes through said position of contactuntil said arc moves toward said first base material and returns back tosaid predetermined position, memory means for storing a change inwelding voltage during said clocking time by said voltage detector andsaid clock device, and wire tip position control means for moving a tipof said welding wire away from said position of contact when an actualaverage voltage during said clocking time falls below a preset firstvoltage level, moving said tip of said welding wire toward said positionof contact when said actual average voltage exceeds a preset secondvoltage level, and maintaining said tip of said welding wire at apreceding position when said actual average voltage falls between saidpreset first voltage level and said preset second voltage level.
 22. Awelding apparatus for butt welding a first base material and a secondbase material with each other, comprising: a welding wire to beintroduced through a welding torch into a groove formed by bringing saidfirst base material, on which a root face and a single groove face havebeen formed, into contact with said second material having a flatsurface, a control device for controlling a position of said weldingwire, a welding power source for feeding welding electric power to saidarea of contact, a current detector for detecting an actual current tobe fed from said welding power source to said area of contact, a clockdevice for detecting a time, at which an arc from said welding wirepasses through a position of contact between an end portion of saidsingle groove face and said flat surface, and clocking a time after saidarc passes through said position of contact until said arc moves towardsaid first base material and returns back to said predeterminedposition, memory means for storing a change in welding current duringsaid clocking time by said current detector and said clock device, andwire tip position control means for moving a tip of said welding wireaway from said position of contact when an actual average current duringsaid clocking time exceeds a preset first current level, moving said tipof said welding wire toward said position of contact when said actualaverage current falls below a preset second current level, andmaintaining said tip of said welding wire at a preceding position whensaid actual average current falls between said preset first currentlevel and said preset second current level.